The present invention relates to liquid coating compositions that provide improved mar and/or scratch resistance and to methods for using the same. More specifically, the improved resistance is achieved by adding modified particles to a film-forming resin.
xe2x80x9cColor-plus-clearxe2x80x9d coating systems involving the application of a colored or pigmented basecoat to a substrate followed by application of a transparent or clear topcoat over the basecoat have become increasingly popular as original finishes for a number of consumer products including, for example, cars and floor coverings such as ceramic tiles and wood flooring. The color-plus-clear coating systems have outstanding appearance properties, including gloss and distinctness of image, due in large part to the clear coat.
xe2x80x9cOne coatxe2x80x9d systems comprising a one coat color layer are applied themselves as the topcoat. One coat systems are frequently used for household appliances, lawn and garden equipment, interior fixtures, and the like.
Liquid coatings are used in many systems, particularly those wherein solvent emissions are permitted. For example, the coating of elastomeric automotive parts is often done by spraying liquid compositions. Many of these compositions are formulated to be flexible so the coating can bend or flex with the substrate without cracking. Because these coatings can result in films that are softer, they may be more susceptible to marring and scratching.
Topcoat film-forming compositions, such as the protective and/or decorative one coats for household appliances and the transparent clearcoat in color-plus-clear coating systems for automotive applications, are subject to defects that occur during the assembly process and damage from both the environment and normal use of the end product. Paint defects that occur during assembly include the paint layer being too thick or too thin, xe2x80x9cfish eyesxe2x80x9d or craters, and under-cured or over-cured paint; these defects can affect the color, brittleness, solvent resistance and mar and scratch performance of the coating. Marring and/or scratching can also occur during assembly due to handling of the parts, and particularly during transit of the parts to the assembly plant. Damaging environmental factors include acidic precipitation, exposure to ultraviolet radiation from sunlight, high relative humidity and high temperatures; these factors can also result in compromised performance. Normal use of consumer products will often lead to marring, scratching and/or chipping of the surface due to contact with hard objects, contact with brushes and/or abrasive cleansers during normal cleaning processes, and the like.
Thus, there is a need in the coatings art for topcoats having good scratch and mar resistance, including those in which flexibility would also be desired.
The present invention is directed to liquid coating compositions generally comprising a film-forming resin in which is dispersed a plurality of chemically modified particles. Particles containing at least one reactive group on their surface are suitable for modification, which is effected by the addition of a moiety that renders the particles more surface active than they would be otherwise; stated another way, the surface tension of the particles is lowered by the present modifications. As a result, the modified particles are more incompatible with the resin than their unmodified counterparts; this is contrary to many art-taught techniques in which particles are treated or reacted with compounds that make them more compatible with, and therefore more easily dispersed in, the resin. Rendering the present particles more incompatible or more surface active causes at least some of the particles to rise to the surface of the cured coating, thus affording enhanced mar and/or scratch resistance.
The particles are typically organic or inorganic particles, or mixtures thereof, and can have an average particle size in the nanometer or micron range. Methods for using compositions comprising modified particles are also within the scope of the invention, as are substrates coated according to these methods.
It has been surprisingly discovered that the incorporation of the present modified particles into a film-forming resin results in coatings having enhanced mar and/or scratch resistance as compared with the same coatings lacking these particles. According to the present invention, coatings can be formulated with these improved mar and/or scratch characteristics without adversely affecting the appearance, viscosity or other mechanical properties of the coatings. Significantly, the flexibility of the cured coating is virtually unaffected by addition of the particles.
xe2x80x9cMarxe2x80x9d and xe2x80x9cscratchxe2x80x9d refer herein to physical deformations resulting from mechanical or chemical abrasion. xe2x80x9cMar resistancexe2x80x9d is a measure of a material""s ability to resist appearance degradation caused by small scale mechanical stress. xe2x80x9cScratch resistancexe2x80x9d is the ability of a material to resist more severe damage that can lead to more visible, deeper or wider trenches. Thus, scratches are generally regarded as being more severe than what is referred to in the art as mar, and the two are regarded in the art as being different. As noted above, mar and scratch can result from manufacturing and environmental factors as well as through normal use. Although mar and scratch are in many respects just differing degrees of the same thing, a coating that improves mar resistance may not be effective in improving scratch resistance, and vice versa. It will be appreciated, therefore, that combinations of modified and unmodified particles and other additives can be employed to give the final coating its desired characteristics. For example, one particle that offers particularly good mar resistance can be coupled with one that offers particularly good scratch resistance. In another example, a treated particle and an untreated particle can be contained to give the desired end performance.